1. Field of the Invention
The present invention relates to a process for producing silicon nitride powder suitable for the production of sintered silicon nitride, useful as a superhard heat resisting material.
2. Description of the Prior Art
A typical known method is the so-called imide or amide decomposition method in which a silicon halide is reacted with ammonia and, then, the resulting silicon diimide or silicon tetramide is heated under an inert gas or ammonia gas atmosphere. This method, however, involves disadvantages in that the reaction of the silicon halide and the ammonia is a very vigorous exothermic reaction. Therefore, control of the reaction is very difficult. Further, a large amount of ammonium halide in the form of fumes is formed as a by-product. The deposition of the ammonium halide by-product on the inner surface of the reactor, the supply nozzles of the starting materials, and the tube wall of the gas outlet cause clogging and various other problems.
A technique has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 54-145400 to eliminate the above-mentioned defects of the imide or amide decomposition method. According to this proposal, silicon nitride powder having a high purity can be obtained. However, this proposed method involves a disadvantage in that the resultant silicon nitride powder is difficult to handle because of a low tap density, due to the large number of needle crystals formed in the calcination step. Therefore, it is necessary to grind the silicon nitride powder and to increase the tap density before the calcination to obtain the sintered product.